The Cancer of the Great Lakes

Mussels are devastating a unique ecosystem.

The first day of June 1988 was sunny, hot, and mostly calm—perfect weather for the three young researchers from the University of…By Dan Egan

The first day of June 1988 was sunny, hot, and mostly calm—perfect weather for the three young researchers from the University of Windsor who were hunting for critters crawling across the bottom of Lake St. Clair. Sonya Santavy was a freshly graduated biologist aboard a 16-foot-long runabout as the whining outboard pushed the boat toward the middle of the lake that straddles the United States and Canadian border.

On a map, Lake St. Clair looks like a 24-mile-wide aneurysm in the river system east of Detroit that connects Lake Huron to Lake Erie, and that is essentially what it is. Water pools in it and then churns through as the outflows from Lakes Superior, Michigan, and Huron tumble down into Erie, then continue flowing east over Niagara Falls into Lake Ontario, and finally down the St. Lawrence Seaway and out to the Atlantic Ocean. The current pulsing through Lake St. Clair is so strong that if you were to hop in an inflatable raft at the top of the lake you’d flush out the other side in about two days—without having to paddle a stroke.

overflow: Invasive mussel shells accrue on the Great Lakes’ shores in piles so high they resemble sand.Royalbroil / Wikipedia

Water rushes so quickly through Lake St. Clair because it is as shallow as a swimming pool in most places, except for an approximately 30-foot-deep navigation channel down its middle. The U.S. Army Corps of Engineers carved that pathway in the late 1950s as part of the Seaway project to allow oceangoing freighters to sail between Lake Erie and the lakes upstream from it. When water levels were low or sediment high, sometimes that channel still wasn’t deep enough, forcing ships to lighten their loads to squeeze through. This often meant dumping water from the ship-steadying ballast tanks—water taken onboard outside the Great Lakes. Water that could be swarming with exotic life picked up at ports across the planet.

As Santavy and her University of Windsor colleagues puttered over a rocky-bottomed portion of Lake St. Clair in the early summer of 1988, she whimsically dropped her sampling scoop into the cobble below. She was hunting for muck-loving worms but figured she’d take a poke into the rocks below because—well, to this day, she still doesn’t know. “I can’t even explain why it popped into my head,” Santavy told me. “I thought—if we get nothing, we get nothing, and I’ll just mark it off that this is not an area to sample.”

Up came a wormless scoop of stones, the smallest of which were not much bigger than her fingertips. But there was something odd about two of those tinier pebbles. They were stuck together. She tried to pull them apart but she couldn’t. Then she realized that one of them wasn’t a pebble at all. It was alive.

Nobody gave it much thought at the time, but in the years following the Seaway’s opening in 1959, species not native to the Great Lakes, ranging from algae to mollusks to fish, started turning up at a rate never before seen. In the Seaway’s inaugural season it was the humpbacked peaclam native to Europe and Asia. In 1962 came Thalassiosira weissflogii, a single-celled alga capable of both sexual and asexual reproduction and, unlike sea lampreys, incapable of being controlled ecosystem-wide by any human measures.

Five more exotic species of algae showed up during the next two years and a tubificid (lake bottom–burrowing) worm native to the Black and Caspian Sea basins arrived in 1965. A water flea from Europe turned up the year after and a European flatworm two years after that. A crustacean native to the Black and Caspian Seas arrived in 1972. Three more exotic species of algae turned up the following year. And the alien organisms continued to arrive, year after year, with an almost metronomic predictability—all the way up to that steamy Wednesday morning on Lake St. Clair in 1988.

The important thing about the zebra mussel is to not consider each one as an individual organism but instead, like a cancer cell, part of a greater scourge.

Santavy showed a graduate student aboard the research boat her living “stone,” its wavy bands having allowed it to blend into the rocks she found it lurking among. It was obvious to both of them that it was some kind of clam or mussel, but the dime-sized mollusk looked like nothing Santavy’s colleague had ever seen. This was odd. He was a graduate student whose job was to study freshwater clams of North America. This made them suspicious enough to bring her specimen back to campus.

When Santavy returned to campus she showed her specimen to the professors in the lab. They were also flummoxed. They sent it to the University of Guelph outside Toronto, where an international mussel expert identified it as Dreissena polymorpha, the zebra mussel. This was not good news. The species, native to the Caspian and Black Sea basins, was well known on the other side of the Atlantic for its ability to fuse to any hard surface, growing in wickedly sharp clusters that can bloody boaters’ hands and swimmers’ feet, plug pipes, foul boat bottoms, and suck the plankton—the life—out of the waters they invade.

The zebra mussel had already colonized rivers and lakes across Western Europe thanks to an extensive network of canals and locks that, like North America’s Seaway, had allowed biological trouble to course through a continent like cancer cells in a bloodstream.

Hungary succumbed to an infestation in 1794, London in 1824. Rotterdam fell in 1826, followed by Hamburg in 1830 and Copenhagen in 1840. The mussels had spread to Switzerland and Italy by the 1970s. And then Santavy’s specimen turned up in Lake St. Clair, some 3,000 miles from the closest known colony.

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A zebra mussel has something of a “foot” that enables it to drag itself across a lake bottom, but even the fastest adult zebra mussel can only trundle along at maybe 14 inches per hour. A pioneering colony of the mussels also could not have inched its way, generation by generation, across the ocean and up the Seaway because the mussels would not have survived the ocean’s salinity or depth. Scientists knew the most plausible way Santavy’s mussel could have made the trip across the Atlantic and into the Great Lakes was in the friendly confines of a freighter ballast tank filled with water from a freshwater or semi-saltwater port.

Ship-steadying ballast used to be solid materials. In the 1800s bars of iron were used to balance schooners in the slave trade and Europe-bound ships laden with tobacco returned to the New World with bricks for ballast. But as freighters shed their sails and wooden hulls, acquired steam engines and grew to titanic proportions, the ships demanded ever-more stabilizing weight, particularly when a vessel was sailing with less-than-full cargo holds, unevenly loaded freight or through violent seas.

Naval architects soon realized water, at just over 8 pounds per gallon, is plenty heavy to function as ballast. More importantly, it does not have to be manually loaded. It can be pumped in or out of the network of tanks tucked under the steel skin of a modern freighter.

But liquid ballast does have one huge drawback—it is anything but dead weight.

The discovery of Santavy’s single shell might have meant little initially to the young researchers who found it. But seasoned ecologists knew the doom it foretold, like radiologists spotting a telltale speck on an X-ray; the important thing about the zebra mussel is to not consider each one as an individual organism but instead, like a cancer cell, part of a greater scourge that metastasizes as fast as currents flow. And, unlike some places in Europe and in the mussels’ native range, North America has no natural mussel predators to keep their numbers from exploding in a manner never before seen.

Biologically contaminated ballast water is the worst kind of pollution because it does not decay and it does not disperse. It breeds.

Each female can produce 1 million eggs per year. Those microscopic offspring—called veligers and as small as a 10th of a millimeter in diameter—are covered with little hairs that help them catch currents and waves and “swim” to new locations during the first few weeks of their lives. The hairs also allow a baby mussel to snag food and begin to grow a shell, which eventually weighs it down and forces the mussel to settle on a lake or river bottom. There, it begins its blind hunt for a hard surface—rocks, glass, pilings, even other mussels—to attach to. Within a year those babies are sending out puffs of their own veligers to establish new colonies.

Distressing as the news was that the zebra mussel had made the jump across the Atlantic, nobody should have been surprised. As early as the late 1800s, naturalists had recognized the zebra mussel as an invasive species juggernaut. “The Dreissena is perhaps better fitted for dissemination by man and subsequent establishment than any other fresh-water shell,” English zoologist Harry Wallis Kew wrote in 1893. “Tenacity of life, unusually rapid propagation, the faculty of becoming attached ... to extraneous substances, and the power of adapting itself to strange and altogether artificial surroundings have combined to make it one of the most successful molluscan colonists in the world.”

stowaways: Every year, an untold number of foreign organisms find their way into the Great Lakes in shipping ballast water.Garitzko / Wikipedia

A final warning came in 1981 when a group of scientists took the time to see what might be lurking in the ballast tanks of overseas Seaway freighters bound for the Great Lakes. They found the tanks were basically floating ecosystems, swarming with life sucked up from ports across the globe. The researchers specifically mentioned zebra mussels among the primary threats to make their way into the lakes by hitching a ride in ballast water, which is often discharged when an overseas ship arrives in the lakes in exchange for cargo. The U.S. and Canadian governments did nothing with the information.

Paul Hebert, director of the University of Windsor laboratory where Santavy worked, told a reporter, “It’s crazy to go on studying and studying this—we have to do something. We’re getting new species in the lake all the time.” The problem was regulators’ hands were tied—by the Clean Water Act itself.

In 1972, Congress overrode a President Nixon veto and approved a sweeping package of amendments to the existing federal water pollution regulations that are known today as the Clean Water Act. This turned the tables by establishing the principle that industry does not have a “right” to pollute and must therefore apply for a permit to do so. To get a permit, a company had to agree to install the best available waste treatment systems for the pollution it discharged. These permits had to be renewed every five years, the idea being that the volume of pollution a business could discharge would be continually ratcheted down as better treatment technologies inevitably evolved over the years and decades. Permit violations carried fines that could total tens of thousands of dollars per day, and major offenders could be sentenced to jail.

The goals of the Clean Water Act were impossibly high—zero pollution discharges by 1985, with an interim target to make all the waters of the United States swimmable and fishable by 1983. The Clean Water Act missed those marks, but the improvements it brought have been immense. In the early 1970s two-thirds of America’s lakes, rivers, and coastal waters were unsafe for fishing or swimming. By 2014 that number had been slashed in half. But the Environmental Protection Agency left one huge loophole in the law the year after it was passed when it expanded an exemption for water discharges from military vessels to all ships sailing in U.S. waters. The agency was likely motivated by the notion that without a ship discharge exemption, its regulators could be on the hook to somehow police millions of recreational boats. Whatever the reason, the agency clearly did not see freighter discharges as a threat.

“This type of discharge generally causes little pollution,” the EPA explained when it published the regulation creating the exemption, “and the exclusion of vessel wastes from the permit requirements will reduce administrative costs drastically.”

But it would cost the Great Lakes dearly. As the zebra mussel infestation of the Great Lakes would make it abundantly clear, biologically contaminated ballast water is the worst kind of pollution because it cannot be fixed by plugging a pipe or capping a smokestack. It does not decay and it does not disperse. It breeds.

Santavy had found only one mussel that summer day in 1988. Everybody knew there had to be more. They just didn’t know how many. Tom Nalepa, then an ecologist with the National Oceanic and Atmospheric Administration, remembers making the three-hour drive from his office in Ann Arbor, Michigan, to London, Ontario, in March 1989 to meet with 11 other scientists about this latest Great Lakes invader. It turned out to be what is today known as the first International Conference on Aquatic Invasive Species, which has become an almost annual event that draws hundreds of researchers from across the globe. But the meeting on this chilly day wasn’t called anything so grandiose. It wasn’t called anything at all. It was just a dozen smart but mystified U.S. and Canadian scientists trying to share everything they knew about an organism that was spreading faster than their ability to read up about it.

The researchers in the room that day, in fact, couldn’t even agree whether to call it a clam or a mussel. Conference host Ron Griffiths of the Ontario Ministry of Natural Resources took the Canadian-nice tack of referring to it as the “zebra mussel clam.” The problem was there was almost no North American literature on the lifecycle of the zebra mussel because, until then, there had been no North American zebra mussels. The scientists had been gleaning what they could from research papers written in Russian, Polish, and Danish just to figure out things like its preferred habitat, its temperature tolerance, and its reproduction rate.

“A lot of the literature I’ve read is in another language, and I can only go as far as the abstract,” Gerry Mackie, a mussel expert from the University of Guelph, confessed at the outset of the conference, a grainy video tape of which has survived more than a quarter century. The researchers turned on a carousel slide projector to look at how far the zebra mussels had spread since Santavy dropped her scoop to the bottom of Lake St. Clair just 10 months earlier. The room got quiet as the wheel stopped on each new image.

• An engine block found on the bottom of Lake St. Clair so encrusted with zebra mussels its piston holes were plugged.• A Coast Guard buoy hauled in from Lake Erie so coated with shells it was unrecognizable.• A Great Lakes beach littered with bleached mussel shells lying open on their sides, like so many little mouths.

Then Griffiths turned on a videotape of a mussel-smothered ferry wharf on the Canadian side of Lake Erie. There were so many shells nobody tried to calculate how densely they coated the wharf’s pilings. It would have been like counting stars from the deck of an ocean freighter on a moonless night. “Man,” Nalepa remembers thinking as he sat with his colleagues around tables littered with coffee cups and jars of zebra mussel specimens. “Nothing is going to be the same. Nothing.”

There was some talk that day about how the plankton-gobbling mussels might affect native fisheries higher up the food web. But the scientists mostly worried about what the mollusks could do to the region’s industries, given their ability to gum up pipes. Researchers quickly realized that water intake pipes used by cities and industries would likely be prime zebra mussel habitat; the hard surface inside a pipe provides an ideal place to attach and the constant flow of water—and the plankton floating in it—make for an easy meal, like a floating buffet. It was already starting to happen.

The chaos this has brought is like nothing the lakes have suffered in their 10,000-year history.

The North American zebra mussel problem was made worse by the fact that they have no worthy predators in the Great Lakes, and in the most heavily infested areas, they soon began to cluster atop each other like gnarled coral at densities exceeding 100,000 per square meter. Each adult mussel, which typically grows no bigger than a nickel, can filter up to a liter of water per day, sequestering inside its hard little shell all the nutrients contained within that water.

By the end of 1989, zebra mussels had turned up all across the Great Lakes, west to Duluth, south to Chicago, and east to the St. Lawrence River below Lake Ontario. A colony was also found near the head of the Chicago Sanitary and Ship Canal that provides a manmade connection between the Great Lakes and the Mississippi River basin. That meant the mussels now had access to a watershed that spans almost half of the continental United States.

But the most ominous mussel development of 1989 made no headlines. Researchers on Lake Erie found what appeared at first to be a slightly different version of the zebra mussel. It was, they would learn two years later, the quagga mussel, named after a subspecies of actual zebras that went extinct in the 1800s. All that remains of the African savanna grazers are seven skeletons, including one on display at University College London. But their molluscan namesake today, in the Great Lakes alone, numbers in the quadrillions.

Zebra mussels proved to be an expensive nuisance indeed for industries and cities that depend on water, costing billions of dollars over the past quarter century to invent, build, and maintain treatment systems that use things like chemicals, heat, and UV light to keep pipes open and water flowing through everything from nuclear power plants to kitchen faucets. Yet the ecological damage wrought by zebra mussels is minor in comparison to their cousin, the quagga mussel. Unlike zebra mussels, which typically aren’t found at depths beyond 60 feet, quaggas have been plucked from waters as deep as 540 feet. This depth tolerance, coupled with the fact that quaggas don’t require a hard surface to attach to, means they can blanket vast swaths of lake bottom inaccessible to zebra mussels. Zebras also only feed during the warmer months. Quaggas filter nutrients out of the water year-round.

In 1992, three years after quagga mussels were discovered in Lake Michigan, zebra mussels still made up more than 98 percent of the lake’s invasive mussel population. By 2005 that relationship had completely flipped, with the quaggas making up 97.7 percent of the invasive mussel population and smothering the deepwater lakebed in a manner zebras never could. Although the waters of Lake Superior lack levels of the shell-building calcium that zebra and quagga mussels require to thrive, the mussel impacts on Lake Michigan have been similarly repeated on the other lakes, particularly Huron and Ontario. The chaos this has brought is like nothing—not even the sea lamprey—the lakes have suffered in their 10,000-year history.

mean mollusks: Together, the zebra (right) and quagga mussels have nearly stripped the Great Lakes of their rich marine life. Dave Brenner, Michigan Sea Grant

The public can comprehend the devastation of a catastrophic wildfire that torches vast stands of trees, leaves a scorched forest floor littered with wildlife carcasses and turns dancing streams into oozes of mud and ash. But forests grow back. The quagga mussel destruction is so profound it is hard to fathom.

“People look at the lake and don’t think of it as having a geography. It’s just a flat surface from above—and from there it looks pretty much the same as it did 30 years ago, but under water, everything has changed,” University of Wisconsin–Milwaukee ecologist Harvey Bootsma says.

The mollusks now stretch across Lake Michigan almost from shore to shore. People might still think of Lake Michigan as an inland sea full of fish. It’s more accurate to think of it as an exotic mussel bed sprawling across thousands of square miles. Lake Michigan’s quagga mass in one recent year was estimated to be about seven times greater than the schools of prey fish that sustain the lake’s salmon and trout. Under some conditions the plankton-feasting mussels can now “filter” all of Lake Michigan in less than two weeks, sucking up the life that is the base of the food web and making its waters some of the clearest freshwater in the world.

Just how much have things changed since quagga mussels took over? A simple way to gauge the amount of plankton in a water body is to take a visual sounding using a crude device called a Secchi disk, named after a 19th-century Italian priest tapped by the one-time Papal Navy to take water clarity readings in the Mediterranean.

The disk is, typically, an 8-inch diameter metal plate with four equally sized alternating black and white wedges, almost like a monochromatic version of the yellow and black nuclear fallout shelter sign. It is lowered by rope into a water body and the point at which it disappears is the water’s Secchi depth. In the late 1980s, before the mussels blanketed the lake bottom, Lake Michigan’s average Secchi depth was 6 meters, or about 20 feet. By 2010 the average depth had tripled and readings began coming in at beyond 100 feet. This nearly vodka-clear water is not the sign of a healthy lake; it’s the sign of one in which the bottom of the food web is collapsing.

One study on southeastern Lake Michigan revealed that by 2009, phytoplankton levels in springtime—the prime plankton-growing time of year—had dropped nearly 90 percent since the mussels took over the lake bottom. It’s probably not a coincidence that the lake’s fish populations have dropped at the same time.

Annual trawling surveys show the lake’s biomass, or overall weight of prey fish, has plummeted from an estimate of about 350 kilotons in the late 1980s to barely 5 kilotons by 2014. And then a federal fisheries survey crew went fishing one warm September day in 2015. The crew from the U.S. Geological Survey was not fishing for flesh. They were fishing for clues. The group was near the end of an annual three-week “prey fish” survey of the bottom of Lake Michigan that has been conducted every year since 1973. The purpose of these autumnal expeditions for bite-sized fish—diminutive species like sculpins, chubs, and alewives—is to check the lake’s gas gauge. This is because the Great Lakes, and Lake Michigan in particular, have been managed primarily for recreational fishermen for the past half century. The more of these little fish that research crews find swimming in the lakes, the more predator fish—hatchery-raised salmon and trout—that can be planted to gobble them up. The whole operation is kind of like the aquatic equivalent of an oversized hunting preserve stocked with trophy sized elk and deer.

The Arcticus team’s surveying is far from a precise means of weighing how many pounds of salmon and trout food is swimming in the lake, though it is an exercise in fishing precision. Year after year, the Lake Michigan researchers, now guided by satellites, hit the exact same seven spots of lake bottom. They start in the waters off Michigan’s Upper Peninsula at the northern end of the lake and then sweep clockwise about 300 miles down its eastern side, across the lake’s U-shaped southern tip near Chicago and then back north along the western shore. Each survey site includes several sweeps of lake bottom at depths varying from beyond 400 feet to less than 60 feet. After each sweep the net is hauled to the surface so the catch can be analyzed.

This nearly vodka-clear water is not the sign of a healthy lake; it’s the sign of one in which the bottom of the food web is collapsing.

This is a lot of lake bottom scraped clean of its fish but it doesn’t even cover a sliver of the overall lake; Lake Michigan has a surface area spanning more than 22,000 square miles and in places plunges into ink-black depths beyond 900 feet deep. Still, from the data—the fish—pulled from the lake on these expeditions, the biologists, assisted by computer models that have been refined over decades, are confident they are able to sketch a good estimate of the number of pounds of prey fish available in the lake in any given year. More than putting a number on the pounds of prey fish in the lake, the surveys are particularly useful in providing an estimate of the relative abundance of prey species from one year to the next. In this sense, the surveys show biologists which direction things are headed. And in recent years, if Lake Michigan’s prey fish population were plotted on a graph like the Dow Jones Industrial Average, the trend would look something like the Panic of 1929.

The first haul that day broke from a Lake Michigan surface that was black as oil because the water was so glass-flat that the sun had neither wave nor ripple off which to glimmer. The catch was dumped into a tub and hauled inside the boat so each fish could be identified, counted, measured and weighed. There wasn’t much work to do. I was expecting at least hundreds of pounds of writhing fish, but the whole catch weighed less than four pounds. We tried again at a slightly shallower depth. And then again, and again and again, and the net kept coming up with close to nothing.

“Gosh,” the expedition leader said after one sweep of the lake bottom netted a single fish the size of my pinky.

“That’s just embarrassing,” said another crew member.

The Arcticus headed toward shore at the end of the day with a total catch that could fit in my first grade daughter’s school backpack. I asked the expedition leader if he thought we just had bad luck: “What can I say!” he said with a pained smile and a shrug. Then he shrugged harder, like a sitcom character, scrunching his shoulders and holding his hands out, palms up. So I asked him if he had seen enough to say Lake Michigan’s prey fish are crashing, particularly the alewives that sustain the salmon fishery. Such a crash has already happened on Lake Huron.

“I wouldn’t say we’ve crashed,” he finally said, “but at least we’re in the process of crashing, you might say.”

hidden costs: The U.S. once had high hopes for Great Lakes shipping lines, but now their commercial benefit is almost negligible.Visual Telling of Stories

People really don’t grasp what has happened here,” Bootsma explained to me on a frigid early November day as he strapped on a scuba tank, climbed over the back of the boat and plunged to the lake bottom 30 feet below. He was only about 800 yards off the beach of a popular park in the leafy Milwaukee suburb of Shorewood. But he might as well have landed on another continent because under the surface Lake Michigan bears little resemblance to the freshwater wonder that left early European explorers awestruck with its teeming herring, trout, sturgeon, perch, and whitefish. Down below, the lake is pretty much only home to the invasive round goby—another Seaway interloper that arrived just a couple of years after the mussels and is also native to the Caspian and Black Sea region—who thrive on the invasive mussels amid a shin-high forest of a nuisance seaweed-like plant called Cladophora, which needs three things to thrive: sunlight, nutrients, and a hard surface.

The mussels have provided all three. Their plankton-stripping ability has dramatically increased the depths to which sunlight can penetrate. Their shells provide a surface on which the seaweed can grow and the mussels’ phosphorus-rich excrement fuels the plant’s growth. The result is an endless forest of brilliantly green, hair-like tendrils swaying in the current, invisible to anyone on shore—until relatively small amounts of it break off, wash ashore and, along with the mussels it has attached to, rot.

The septic-smelling muck plagues some of the lakes’ most spectacular shorelines, including Sleeping Bear Dunes, a 35-mile stretch of federally protected coastline on Lake Michigan’s eastern shore. At Newport State Park on the other side of the lake, Wisconsin’s only wilderness park, the sludge can get shin-deep. A park employee in recent years has taken to keeping a laminated picture of the beach from the pre-quagga days, to show visitors how pleasant her sandy shores used to be. But the mess on the beach is nothing compared to what’s happening all across the lake bottom.

“What people see on a beach is just the tip of the iceberg,” said Bootsma. “You’ve got maybe a few thousand square feet of it on the beach—but just offshore, out in the lake, you’ve got thousands upon thousands of acres of it.”

It’s not just native fish species and summertime beachgoers that have been hit by this biological pollution. Invasive species can have effects just as toxic as the nastiest chemicals concocted in a lab. A textbook example is the botulism outbreaks that have been killing tens of thousands of birds on Lakes Michigan, Erie and Ontario in recent years. It’s a crash course in living pollution that is as simple as it is frightening:

• Invasive mussels have increased water clarity.• That has led to a bloom in the sunlight-loving Cladophora that eventually dies and burns up massive amounts of oxygen as it decomposes on the lake bottom.• That has opened the door to botulism-causing bacteria that thrive in oxygen-starved environments.• The invasive mussels, many biologists believe, suck up those bacteria and are, in turn, eaten by gobies.• The poisoned gobies become paralyzed and are easy prey for birds like loons, grebes, and gulls.• The birds die.

This is not a rare occurrence. Biologists estimate more than 100,000 dead birds—including bald eagles, great blue herons, ducks, loons, terns, and plovers—have piled up on Great Lakes beaches since the botulism outbreaks turned rampant in 1999.

In 1993 the U.S. Coast Guard made exchanging ballast water with mid-ocean saltwater mandatory, yet wave after wave of new invasions kept rolling into the Great Lakes. The problem was about 90 percent of the ships arriving in the Great Lakes from foreign ports at that time came fully loaded with cargo and therefore did not officially carry any ballast water, and the new law exempted those ships from the ballast exchange requirement. But just because a captain declared his vessel to be ballast-free did not mean his tanks were empty. Most tanks still carried loads of sludge—up to 100,000 pounds of it—along with thousands of gallons of residual ballast puddles that cannot be emptied with a ship’s pumps.

Subsequent studies revealed these muddy puddles swarmed with millions of organisms representing dozens of exotic species that had yet to be found in the Great Lakes. Furthermore, the life lurking in this muck had an easy escape from the bowels of a ship once a captain unloaded cargo at his first Great Lakes port of call and then filled his ballast tanks before steaming to the next Great Lakes port. That ballast—and any organisms churned up from the sludgy bottoms during the cross-lakes voyage—could then be discharged when the captain swapped it for cargo at the next port.

Between 1990 and 2008, at least 27 new exotic species were discovered in the lakes. The pace peaked around 2005, when a foreign organism was being detected, on average, about once every eight months. The official tally is there are now at least 186 nonnative organisms swimming or lurking in the Great Lakes. Not every one of these life forms can be defined as “invasive” because some were planted and are considered by many to be an asset to the region (the salmon and some stocks of exotic trout, for example) and others apparently exist in their new home without any discernable negative environmental or economic impact. But sometimes it takes two foreign organisms working together to cause utterly unpredictable trouble, as in the apparent case with invasive mussels and gobies working together to trigger botulism outbreaks in native birds. These kinds of chain reactions can take years or even decades to unfold, and they make it impossible for biologists at any point to know which introductions will be harmless and which will become troublesome, if not disastrous.

In 2008 the U.S. Seaway operators began requiring all Great Lakes-bound overseas vessels to flush even their “empty” ballast tanks with mid-ocean saltwater. No new exotic organisms have been found in the Great Lakes since, a point shipping industry advocates tout. Although it is generally agreed that mid-ocean flushing is an excellent first step toward closing the door to new ballast water invasions, it is also generally agreed upon that ballast water disinfection systems similar to sewage treatment plants must be used by ships to provide ample protection for the Great Lakes and other U.S. waters. It’s a numbers problem.

Even if a ballast flush kills or expels more than 99 percent of hitch-hikers, ships arriving in the Great Lakes from ports around the globe are still far from sterile. One study has shown that a single freighter ballast tank can harbor some 300 million viable cysts of primitive dinoflagellates, which scientists dub the “cells from hell” because they can produce a deadly neurotoxin. So a flush that eliminates 99 percent of that ballast tank’s inhabitants could still carry 3 million potential invaders. That’s just one ballast compartment, and that’s just one species.

The EPA has a watch list of dozens of species that remain a risk to invade the lakes in ship ballast tanks, despite the saltwater flushing requirements. On that roster of the unwanted is the notorious Dikerogammarus villosus, otherwise known as the killer shrimp. The murderous crustacean got its name because it makes a mess of the ecosystems it invades by destroying its prey with vise-like jaws and then leaving its victims for dead, often without swallowing a bite. These shrimp, which can grow bigger than an inch, have been spreading through Europe’s canal system for decades. Studies show they have a salt tolerance just slightly below what is typically found in the ocean, which means that if the shrimp get sucked into the ballast tank of a ship sailing for the Great Lakes, there is perhaps only a membrane-thin margin of protection against another wave of destruction in the lakes.

There are, not surprisingly, other pathways for species to invade the Great Lakes. Fishermen from other watersheds can dump their bait buckets. Aquarium owners can dump their pets. Anglers hoping to “improve” their fishing prospects can intentionally plant exotic fish. But contaminated ballast water from overseas vessels has been, by far, the dominant pathway for Great Lakes invasions since the Seaway opened.

Contaminated ballast water is also not just a Great Lakes problem. It has been linked to a cholera outbreak that killed more than 10,000 people in South America two decades ago. It is why Chinese mitten crabs and Asian clams are ravaging what’s left of the native species in the heavily invaded San Francisco Bay and a cantaloupe-sized snail from Asia called the veined rapa whelk is creeping across the bottom of the Chesapeake Bay. But guarding the Atlantic and Pacific coasts from these invaders is a far more daunting task, both because of the volume of overseas freighter traffic servicing the Pacific, Atlantic, and Gulf coasts and their geographical vastness.

The Great Lakes themselves are wrapped by some ten thousands of miles of shoreline, every mile of which is vulnerable to the biological mischief ferried in by overseas ships. But unlike on the Atlantic, Gulf, or Pacific Coasts, there is, literally, a door through which every foreign Seaway ship must pass before arriving at the dozens of ports rimming the Great Lakes’ shores: the St. Lambert Lock at Montreal. Every ship—and every one of the Great Lakes invaders it may be carrying—has to squeeze through this 80-foot-wide pinch point. Stop the overseas ships, known regionally as “salties,” and you stop their ballast invasions. “Off-load the cargo in Nova Scotia and ship it down through rail,” an exasperated former Chicago Mayor Richard M. Daley once told me. “That will protect the Great Lakes forever. That will protect local and state governments from spending hundreds of millions of dollars.” He is not alone.

Conservationists agree this low-tech solution for the Great Lakes could prove far cheaper than installing on each ship ballast treatment systems that could cost well over $1 million.

Nobody disputes that Great Lakes shipping is a big, critically important business. But the vast majority of the cargo moved on the Great Lakes Seaway—things like salt, iron ore, coal, and cement—is carried by U.S. and Canadian domestic freighters that tote those bulk materials from one Canadian or U.S. port to another. Overseas freighters in a normal year account for 5 percent or less of the overall tonnage moved annually on the Great Lakes and Seaway. These ships typically carry inbound shipments of foreign steel and outbound loads of grain, which in 2011 accounted for less than 2 percent of total grain exports from the United States and Canada. And the total tonnage carried annually by salties sailing into and out of the Great Lakes has been slipping for decades. It is now roughly equal to what could be hauled by a single daily inbound and outbound train from the East Coast.

Dan Egan is a reporter at the Milwaukee Journal Sentinel. He has twice been a finalist for the Pulitzer Prize, and he has won the Alfred I. duPont–Columbia University Award, John B. Oakes Award, AAAS Kavli Science Journalism Award, and J. Anthony Lukas Work-in-Progress Award.

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